It sounds like the Subcontractor’s CE notification is made on legitimate grounds.
Regarding concurrency (NB. there is no such concept in the NEC, and even if there was it is not simple as the two events should have started occurring at the same time, with the “same time” being a topic by itself; the Contractor probably refers to “overlapping” delay), any such matter would have to be dealt with within the assessment process, following acceptance of the CE and the Subcontractor’s quotation for time and cost impact.
See below some very helpful resources on the matter of concurrency, in relation to the NEC contracts: